An infant has microcephaly, hypotelorism, a single midline nostril, and imaging showing fusion of the cerebral hemispheres with a single ventricle. Karyotype reveals trisomy 13. The primary embryological failure is:

• A Failure of closure of the rostral neuropore
• B Failure of cleavage of the prosencephalon into two cerebral hemispheres ✓
• C Failure of migration of cranial neural crest cells
• D Failure of formation of the corpus callosum

Explanation

Holoprosencephaly results from failure of the prosencephalon (forebrain vesicle) to divide into two cerebral hemispheres, producing a single ventricle, fused basal ganglia, and characteristic midline facial defects such as hypotelorism and cyclopia. Sonic hedgehog signalling is implicated. Rostral neuropore failure causes anencephaly, which is distinct, and isolated agenesis of the corpus callosum does not produce hemispheric fusion.

Reference: Langman's Medical Embryology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.